Support thrown expectations in DStreamProp.forall

[juanrh]

Currently only functional expectations work as expected, see https://etorreborre.github.io/specs2/guide/SPECS2-3.6.2/org.specs2.guide.Structure.html#unit-specification

Anyway functional expectations are recommended in ScalaCheck, while -thrown expectations are discouraged https://groups.google.com/forum/#!topic/scalacheck/Ped7joQLhnY